Alite decomposition

Disappearance of ferrite phase, iron transformed to metallic state, clinker color changes to white, alite decomposition structures vanish Extreme reducing conditions (Woermann, 1960)... 

Alite decomposition into secondary belite Overburning (Dorn, 1979)... 

Myrmekitical intergrowth of belite in alite, pseudomorphous free lime, and remnants of nondecomposed alite Decomposition via slow cooling sometimes accelerated by foreign ions, excess alkalies, embedded Fe, and moderate reducing conditions (Gille and others, 1965)... 

The primary phases all contain impurities. In fact these impurities stabilize the stmctures formed at high temperatures so that decomposition or transformations do not occur during cooling, as occurs with the pure compounds. For example, pure C S exists in at least six polymorphic forms each having a sharply defined temperature range of stability, whereas alite exists in three stabilized forms at room temperature depending on the impurities. Some properties of the more common phases in Portland clinkers are given in Table 2. 

All the effects described above indicate that rapid cooling is desirable the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind. 

Subsequent elimination of water and rearrangement of Ca, Si and O lead to formation of the products phases Ca SiO and CaO. Glasser [22] is stating also that the presence of molten sulphates is enhancing the decomposition of C3S. They are formed globules on alite surface leading to its decomposition. 

The sulphate attack with C-S-H gel decomposition and thaumasite formation is particularly harmful for the durability of concrete, because it occurs with the destraction of the most important binding constituent of concrete and leads to the collapse of this material. Because of the relatively low temperatures of thaumasite formation, the concrete foundations, and elements of the underground sewage systems, as well as the concrete road elements are especially susceptible for this type of destructioa In order to prevent thaumasite formation the classic approach should be applied, first of all the permeability of concrete should be reduced, and the use of cement with mineral additions should be considered. Bensted [273] suggested the lowering the C3A and alite content in cement. 

As it is known, that the heat consiunption of the clinkering process is mainly due to the calcium carbonate decomposition and the long period (about 20 min) of clinker heating in sintering zone at maximirm temperamre, to assirre full alite formation. Therefore the energy consirmption increases with lime saturation factor (see Chap. 2). 

Alite crystal chemistry was discussed in a paper by Ono (1974) in which he described changes in the atomic structure of alite in response to such variables as solid solution, exsolution, thermal vibration, states of disorder, inversion, and partial decomposition. 

Tiny belite crystals formed as a surficial decomposition of the alite during slow cooling have always been clear in the writer s observations secondary belite (formed out of the matrix during cooling) is also clear, perhaps because these crystals have little to exsolve. Research is needed on this point. Amoeboid crystals are rarely colored dendritic crystals have never been observed to be colored in the writer s experience. Ono s belite color interpretation seems most applicable to properly formed crystals. 

Dusty clinker high porosity, breaded nodules, agglomerated fine particles, alite-rich, large alite crystals, and relatively scarce liquid phase (a) Decomposition of outer clinker shell and concentration of liquid phase in clinker core (Allegre and Terrier, 1960) (b) Recycling of precipitator dust, reducing zones, lack of AljOj in raw slurry (Hofmanner, 1973)... 

Increase in alite percentage and crystal size, decrease in belite percentage, decomposition of C3A Increase in burning temperature and time with = 1450°C to 1750°C and time up to 800 seconds (Butt, Timashev, and Starke, 1973)... 

Decomposition of alite Long annealing time or repeated heat treatment at lower temperatures, approximately 1200 =0 (Ghosh, 1983)... 

Decomposition of alite to belite and free lime abnormally large belite and alite Slow cooling in kiln (HofmSnner, 1973)... 

Pseudomorphic crystals after primary alite contain fine inclusions or margins of ferrite, belite, and free lime Moderate reduction, and introduction of iron into alite lattice resulting in instability and decomposition (Woermann, 1960)... 

Decomposition of alite to secondary belite and free lime decrease in compressive strength possible abnormal expansion Reducing conditions, coarse coal particles falling into raw mix (Hawthorne, Richey, and Demoulian, 1981)... 

Decomposition of alite varied between 10% to 85% exceptions in low-Fe or no-Fe clinker Lab experiment with reducing conditions, slow cooling (5°C per min, 90 min from 1450°C to 1000°C) (Woermann, 1960)... 

High reflectivity of ferrite, eutectic intergrowth of ferrite and aluminate, no decomposition of alite Normal, oxidized clinker (Woermann, 1960)... 

The use of X-ray diffraction (XRD) data can clearly be supportive to microscopy in almost all materials investigations. One obvious advantage of XRD analysis is that it is a reliable mode of identification and quantification for almost all of the major phases commonly seen in raw feed, clinker, and cement. However, XRD analysis suffers one major disadvantage it cannot tell you where, in the examined mass of clinker, the belite is located or how the belite is related to alite. XRD analysis can indicate the presence of belite and, with some calibrated standards, the amoimts of belite polymorphs, but only the microscope can tell you that the belite occurs mostly as decomposition of alite crystals or that the belite occurs only as inclusions in alite. XRD analysis may indicate the presence and quantity of more than... 

Woermann, E., "Decomposition of Alite in Technical Portland Cement Qinker," Fourth International Symposium on the Chemistry of Cement, Paper II-S8, Washington, D.C., 1960, pp. 119-129. 

The thermal decomposition of calcium carbonate is a highly endothermic reaction, and even though the formations of both dicalcium and tricalcium sihcate from CaO and Si02 are exothermic, the overall formation process of these compoimds from CaCOj and Si02 remains endothermic. A comparison of both reactions reveals that the overall chemical enthalpy of C3S formation is distinctly higher than that of C2S (1268 J/kg for C2S and 1770 J/kg for C3S). Thus the potential exists to reduce the energy consumption in the production of Portland clinker by increasing its behte content at the expense of alite. 

Woermann, E. Decomposition of alite in technical portland cement clinker. — Proc. IV. Intern. Sympos. Chem. Cem., Washington 1960, Bd. 1, S. 119. 

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